Method for Fabricating Equal Height Metal Pillars of Different Diameters

ABSTRACT

A process to form metal pillars on a flip-chip device. The pillars, along with a layer of solder, will be used to bond die pads on the device to respective substrate pads on a substrate. A photoresist is deposited over the device and first openings in the photoresist are formed. Metal layers are formed by electroplating metal into the first openings for a first time period. Then the photoresist is patterned to form second openings having a smaller diameter than the first openings. Narrow pillars are formed by electroplating metal into the second openings for a second time period during which the metal is also added to the metal layers in the first openings to form wide pillars having substantially the same height as the narrow pillars. The photoresist is then removed along with conductive layers on the device used as part of the plating process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of U.S. provisional patent application No. 61/952,963 filed 14 Mar. 2014 as attorney docket no. L14-0124US1, the teachings of which are incorporated herein by reference, and the subject matter of this application is related to U.S. patent application Ser. No. ______, filed concurrently herewith as attorney docket no. L14-0124US1, titled “Method for Fabricating Equal Height Metal Pillars of Different Diameters”, the teachings of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to semiconductor packaging technology generally and, more specifically, to a process for forming copper pillars, and a solder layer thereon, on semiconductor devices for flip-chip bonding to it substrate.

2. Description of the Related Art

Copper pillars are a widely used technique for electrically interconnecting a flip-chip semiconductor device or “chip” to conductors on an organic-based substrate, such as a thin (less than one millimeter thick) glass-epoxy board, because copper pillar interconnects have superior geometric control, higher density, and electrical performance relative to solder bump interconnects. The copper pillars on the device's die pads, formed by selectively plating copper onto the die pads, connect to the substrate's substrate pads by using a solder layer between each pillar and the respective substrate pad to join the copper pillars to the substrate pads. Plating is usually used to form the solder layer onto the ends of the copper pillars.

To bond a flip-chip device to a substrate, the device and substrate are brought together and heated until the solder on the ends of the copper pillars melts and wets the substrate pads on the substrate, each pillar and solder combination forming a “joint”. Then the device-substrate combination is cooled down and the solder solidifies to bond the device to the substrate, forming a bonded device-substrate structure or “package”.

In order to insure all substrate-to-die joints are formed during bonding, all of the copper pillars and solder layers on the die before heating are to have the same nominal height. In addition it is generally desirable for all of the joints to have substantially the same diameter. However, having joints with the same diameter might not be desirable in all instances. For example, for carrying a large number of high-speed signals between the chip and the substrate, it might be desirable to use thinner than “normal” diameter joints spaced to provide a high density of signal paths while at the same time providing a desired transmission line characteristic impedance between the joints, e.g., 50 or 100Ω. In other instances where a large current is to be carried by a joint, e.g., a power supply connection, electromigration might with time cause failure of a joint with a normal diameter. To address the high current problem, multiple joints with a normal diameter are placed in parallel or one or more of the joints are formed with a larger or wider diameter than a “normal” joint so that the current density in each joint is less than a maximum amount that would otherwise cause the joint to fail from electromigration. However, using a conventional plating process to make joints with different diameters with substantially uniform height has been problematic. For a given electrochemical plating process and plating bath solution, the mass or volume per unit of time of the plated material is essentially a constant except for any local variations in the bath current density or concentration of all of the plating species in a particular plating bath. As a result, using a conventional electroplating process to form different diameter joints will result in a device with smaller diameter joints that are taller than adjacent larger diameter joints. The uneven joint height might not allow the shorter joints on the device to be completely attached, if at all, to their respective substrate pads, while all of the taller joints will be completely attached, thus causing the completed package to be inoperable or prone to high rates of failure in the field. Further, any warpage of the substrate might exacerbate this situation, possibly increasing the number of partial or incomplete joints.

SUMMARY OF THE INVENTION

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

Described embodiments include method comprising the steps of providing a wafer having a plurality of flip-chip devices, each flip-chip device having a plurality of die pads thereon; depositing a first layer of photoresist on the wafer; patterning the first layer of photoresist to form a first plurality of openings therein, each of the first openings having a first diameter and exposing a first set of die pads; plating metal into the first openings to form a first metal pillar in each of the first openings for a first time period; patterning the first layer of photoresist to form a second plurality of openings therein, each of the second openings having a second diameter and exposing a second set of die pads; and plating, metal into the first and second openings for a second time period to add to each of the metal pillars in the first openings and form a metal pillar in each of the second openings. The first diameter is greater than the second diameter, and the first set of die pads is different from the second set of die pads. The first and second time periods and plating conditions are chosen such that, after plating metal into the first and second openings for a second time period, each of the pillars in the second openings has substantially the same height as a pillars in each of the first openings.

BRIEF DESCRIPTION OF THE DRAWINGS

Other embodiments of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which like reference numerals identify similar or identical elements. The drawings are not to scale.

FIG. 1 is a cross-section of a flip-chip device bonded to a substrate using copper pillars and solder of different diameters in one embodiment of the invention;

FIG. 2 is a cross-section of one pillar and solder layer of FIG. 1 prior to bonding;

FIG. 3 a flowchart illustrating an exemplary process for forming metal pillars of different diameters and uniform height on a flip-chip device and then bonding the device to a substrate according to one embodiment of the invention; and

FIGS. 4-9 are diagrams illustrating the various steps in the process described in connection with FIG. 3.

DETAILED DESCRIPTION

Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation”.

As used in this application, the word “exemplary” is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, use of the word exemplary is intended to present concepts in a concrete fashion.

The term “or” should be interpreted as inclusive unless stated otherwise. Further, elements in a figure having subscripted reference numbers, e.g., 100 ₁, 100 ₂, . . . 100 _(K), or 100 _(A), 100 _(B), etc. might be collectively referred to herein using a single reference number, e,g 100.

It should be understood that the steps of the exemplary methods set forth herein are not necessarily required to be performed in the order described, and the order of the steps of such methods should be understood to be merely exemplary. Likewise, additional steps might be included in such methods, and certain steps might be omitted or combined, in methods consistent with various embodiments of the present invention.

Also for purposes of this description, the terms “couple”, “coupling”, “coupled”, “connect”, “connecting”, or “connected” refer to any manner known in the art or later developed in which energy or a signal is allowed to be transferred between two or more elements, and the interposition of one or more additional elements is contemplated, although not required. Conversely, the terms “directly coupled”, “directly connected”, etc., imply the absence of such additional elements.

The present invention will be described herein in the context of illustrative embodiments of a process to form metal pillars on a flip-chip device that will be bonded to a substrate by joining die pads on the flip-chip device to substrate pads on the substrate using joints of metallic pillars, such as copper pillars, and solder. The pillars and the solder on them are formed by electroplating a metal onto the die pads. According to Faraday's Law, the total amount of metal deposited on a workpiece by electroplating is proportional to the current passed through the workpiece in an electroplating bath the device is immersed in, and the amount of time the current is applied, i.e., the amount of charge (in coulombs) applied to the workpiece. However, the rate at which each pillar is formed is inversely proportional to the surface area of the pillar. Thus, the wider pillars grow more slowly than narrow pillars. To form narrow and wide pillars having approximately the same height, the wide pillars are first partially formed before forming the narrow pillars by using a photoresist layer with two different diameter openings. First, a photoresist layer is deposited on a device and the photoresist is patterned to form a first plurality of openings therein, each of the first openings having a first diameter and exposing a first set of die pads on the device. Next, pillar metal (e.g., copper) is plated into the first openings to form a first metal pillar in each of the first openings for a first time period. Then the photoresist is patterned to form a second plurality of openings therein, each of the second openings having a diameter smaller than the first diameter and exposing a second set of die pads. Then pillar metal is plated into the first and second openings for a second time period to add metal to each of the metal pillars in the first openings and form a metal pillar in each of the second openings. The first and second time periods and plating conditions are chosen such that, after plating metal into the first and second openings for the second time period, each of the pillars in the second openings has substantially the same height as the pillars in each of the first openings. As described below, the solder layers might also be formed by plating solder into the openings and in conjunction with the formation of the pillars.

FIG. 1 is a cross-section of a flip-chip device 102 bonded to a substrate 104 to form a flip-chip package 100 in accordance with an embodiment of the invention. Die pads (not shown) on the device 102 are bonded to substrate pads 114 on the substrate 104 using multiple conductors or joints 106. The device 102 might be formed from silicon, gallium arsenide, indium phosphide, or another semiconductor material suitable for the desired function of the device 102. The substrate 104 might be formed from a glass-epoxy (commonly known as FR-4), polytetrafluoroethylene (PTFE), polyimide, ceramics, silicon, glass, another insulating material suitable as a substrate, or a combination of these materials. Typically, the thickness of the substrate 104 is less than two millimeters and might be as thin as 50 microns (μm). The lateral dimensions of the substrate 104 are typically larger than that of the device 102.

In this example, the joints 106 are arranged with narrow joints 106 _(N) on the right side of the package 100 and wide joints 106 _(W) on the left side of the package 100. Generally, power and ground are supplied to the device 100 using the wide joints 106 _(W) on the left side of the device 102 and high-speed signals are carded by the narrow joints 106 _(N) on the right side of the device 102. It is understood that in various instances, the wide joints carry the high-speed signals and the narrow joints supply power and ground to the device 102. Further, the positions of the wide and narrow joints are greatly simplified for illustrative purposes; typically hundreds of joints are present and power/ground interconnections are generally made in the center of the device 102 while high-speed signals are generally carried by joints near the periphery of the device 102.

Each joint is formed from a metallic pillar 108, such as copper, and a layer of solder 110. For the proper bonding of all the joints between the device 102 and substrate 104, the height of all of the pillars 108 and solder layers 110 prior to bonding should be of uniform height, otherwise when the device 102 is bonded to substrate 104, a gap between some of the solder layers 110 and the respective substrate pad 114 might be so large that during reflow when the device and substrate are heated sufficiently for the solder to melt, balling-up by the solder on the end of the pillar (caused by surface tension of the molten solder) is insufficient to bridge the gap so that the solder does not wet the substrate pad and no electrical/mechanical joint is made.

Referring to FIG. 2, one of the joints 106 in FIG. 1 is shown as the joint appears prior to bonding the device 102 to the substrate 104. As discussed above, a joint 106 is formed from a copper pillar 108 and a layer of solder 110. The copper pillar is formed on a die pad 212 that has, in this embodiment, three layers. A contact layer 214 is typically in or on the surface of the device 102, is made of copper or aluminum, and connects to circuitry (not shown) within the device 102. Over the contact layer 214 is a layer 216 of titanium or titanium-tungsten that operates as a barrier/glue layer. Next, a strike layer 218, typically of the same metal as used to form the pillar 108 (e.g., copper). As will be explained in more detail below, the layers 216 and 218 are sputtered onto the device 102 and are typically less than 100 nm thick. Not shown are one or more conventional passivation layers (e.g., silicon nitride or alternating layers of silicon nitride and silicon dioxide) on the surface of the device 102 that has openings therein to expose the layer 214 and serves to protect the device from contaminants and physical damage.

On substrate 104 is a respective one of the substrate pads 114, also typically made of copper, shown aligned with the die pad 212. The substrate copper pad might be coated with another metal such as tin, silver, a nickel-gold eutectic, or solder.

The copper pillar 108 has a height of H_(P) and the solder layer 110 has a height of H_(S) (before melting), and both have an approximate diameter D. The height of the joint is H_(P)+H_(S) so that the total height of the pillar and solder is H. In various embodiments, the height of the joint prior to melting ranges from 5 μm to 130 μm. In one embodiment, the copper pillars have a diameter ranging from approximately 20 μm to approximately 80 μm, a height H_(P) of 20-70 μm, and the solder layers, prior to melting, have a height H_(S) of 10-60 μm so that the total height is approximately 80 μm and might range from 5 μm to 130 μm. However, it is understood that the ratio of the height of the copper pillar to the height of the solder layer before melting can range from 1:10 to 100:1 and the pillar diameter can range from 5-150 μm.

Assuming a possible pillar diameter accuracy of +/−1 μm for openings less than 10 μm, +/−2 μm for openings less than 30 μm, +/−5 for openings less than 50 μm, and +/−7 μm for openings less than 100 μm, and +/−8 μm for openings less than 150 μm, then for the following exemplary ranges in opening diameters, it might be desirable to perform two separate pillar plating steps in accordance with the disclosed embodiments when the exemplary percentage difference between the narrow and wide openings is at least that in the following table:

Minimum % diameter

Both opening diameters two separate plating steps <60 10 60 < dia. < 100 7 100 < dia. < 150  5

indicates data missing or illegible when filed

However, it is understood that while the above percentage differences and ranges in opening diameters are merely exemplary, two separate plating steps might still be used were the percentage difference between the narrow and wide openings is less than the above-described amounts.

FIG. 3 is a flowchart illustrating an exemplary process 300 for forming the pillars 108 and the solder layers 110 onto device 102 according to one embodiment of the invention. Beginning with step 302, a wafer containing multiple devices 102 is provided, each device having contact layers 214 thereon. For simplicity and because of the scale of a joint compared to that of a wafer, the process 300 is described below in the context of a single flip-chip device 102 but in practice the process steps are done at a wafer level because a wafer (not shown) comprises multiple flips-chip devices and all of the devices in the wafer are processed at the same time. It is not until an individual device is attached to a substrate (step 324) that the described steps are applied to a device individually.

Next step 304, the barrier/glue layer 216 and the strike layer 218 are deposited over the device 102. These two steps are illustrated in FIG. 4. Here, a partial cross-sectional view of the device 102 of FIG. 1 is shown but with just four contact layers 214, the two on the left being wider than the two on the right. Over the contact layer 214 are two layers 216 and 218, shown here for simplicity as one layer. As discussed above, these layers are each about 100 nm thick and are sputtered, onto the device 102 although other techniques might be used to deposit the layers 216, 218, such as by evaporation. Because the barrier layer and the strike layers are conductive, the combined layers 216, 218 also serve as an electrode when the pillars and solder layer are later deposited by electroplating. While the die pad 212 in FIG. 2 is shown having three separate and laterally defined layers, for purposes of describing the embodiment in FIGS. 4-9, a contact layer 214, along with the metal layers immediately above the contact layer, are referred to herein as either a wide die pad 212 _(W) or a narrow die pad 212 _(N) as illustrated in FIG. 4. The widths of the contact layers 214 and die pads 212 are illustrative (e.g., wider pillars on wider die pads 212 _(W) and narrow pillars on narrow die pads 212 _(N)) and all of the die pads might be the same size or have different sizes. However, the size of a die pad 212 should be at least as wide as the pillar formed thereon.

Returning to FIG. 3, in step 306 a first photoresist layer is deposited across the device 102 and is photolithigraphically patterned using conventional techniques to form openings having a wide diameter in the photoresist over the wider die pads. This is shown in FIG. 5 where wide openings 502 are shown in photoresist 504 over the wide die pads 212 _(W) and the narrow die pads 212 _(N) remain covered by the photoresist 504. The width of the later-formed pillars will be approximately equal to the width of the wide openings 502. Part of the photoresist 504 is also removed to expose the conductive layers 216, 218 and an electrode 506 is shown contacting the conductive layers 216, 218 to provide to current path needed for electroplating. This electrode is not needed if the pillars and solder layer are formed using electroless plating.

In step 308 of FIG. 3, the pillars 108 _(W) are formed by plating metal onto the exposed portions of layer 216, 218 of the die pads 212 _(W). Generally, this is accomplished by submersing the device 102 in a plating path (not shown) containing an aqueous solution of the metal being plated, e.g., copper sulfate, copper cyanide, nickel sulfate, etc. and electroplating the metal into the openings and onto the exposed die pads by applying current to the electrode 506 for a first time period that will result in the deposition of a first layer 506 of pillar metal to a first height, here H₁, in wide openings 502 as shown in FIG. 5. Then the device 102 is removed from the plating bath and cleaned.

Next, in step 310, the first photoresist is again patterned but this time forming narrow openings in the first photoresist to expose the narrow die pads while leaving the wide openings intact and the metal layers in the wide openings exposed. Then, in step 312, the wafer is returned to the plating bath and the deposition of the pillar metal into the wide and narrow openings occurs for a second time period until the wide pillars formed in the wide openings and narrow pillars formed in the narrow openings have approximately the same height. Thus, the narrow pillars are formed solely during the second plating step (step 312) while the wide pillars are formed during both the first plating step (step 308) and the second plating step. Because the metal being plated will be deposited more quickly in the narrow openings than in the wide openings, the first time period and the second time period are chosen so that the narrow pillars will reach a second or desired height at approximately the same time the wide pillars reach the desired height during the second plating step. This is illustrated in FIG. 6, where narrow openings 602 are formed, exposing narrow die pads 212 _(N). During the second plating step described above (step 312), pillar metal is deposited in opening 502 on top of metal layer 506 to form metal layer 606, layer 606 having a height of H₂, and into openings 602 to form the metal pillars 108 _(N). After the second plating step 312, the combined height of metal layers 506 and 606 will be approximately equal to the height of the narrow pillar 108 _(N), or H₁−H₂≅H_(P). Thus, the combined layers 506 and 606 form the pillars 108 _(W).

As mentioned above, the rate at which a pillar is formed (e.g., in microns per minute) is inversely proportional to the surface area of the pillar. To determine the first and second time periods for steps 308 and 312, respectively, the second time period (t2) is chosen so that the narrow pillars have a second or desired height after the second plating step 312. Assuming that the plating conditions (e.g., plating current) are substantially the same during the first and second plating steps, the first time period, t1, might be calculated as a function of the pillar areas and the second time period:

t1≅t2(A_(W)/A_(S)−1);

where t1 and t2 are the first and second time periods, respectively, A_(W) is the area of each of the wide pillars or that of the wide openings, and A_(N) is the area of the narrow pillars or that of the narrow openings.

It is understood that if the plating conditions are different during the first plating step 308 from that in the second plating step 312, the first time period might be adjusted accordingly. Further, more than two different diameter metal pillars might be made by the above process where the number of plating steps is the same as the number of different diameter metal pillars to be formed and the plating times for the different plating steps are determined substantially in accordance with the above equation.

After the narrow and wide pillars 108 _(W), 108 _(N) are formed, then optional solder layers might be formed on the ends of the pillars. Similar the above-described process to form the pillars, in step 314 the wafer is placed in a solder plating bath for a third time period and solder is plated into the narrow and the wide openings in the first photoresist until the solder layer in the narrow openings reaches a desired height above the narrow pillars and then the wafer is removed from the plating bath. As illustrated in FIG. 7, the wafer having device 102 thereon is placed in another plating bath (not shown) containing the solder to be plated, e.g., stannous tin and lead sulfate, stannous tin and silver nitrate, etc., to deposit the solder layers 706 in the wide openings 502 and the solder layers 110 _(N) in the narrow openings 602. The height of the pillars 108 _(N) and 108 _(W) are shown as H_(P), the height of the solder layer 110 _(N) on pillars 108 _(N) is shown as H_(S), and the total height of the pillar and solder layer is height H, the same as that shown in FIG. 2. For convenience, the top of the photoresist 504 is shown coincident with the total height H of the pillar and solder layer but the height H might be below the top of the photoresist 504.

As shown in FIG. 7, the solder layer 706 will be significantly shorter than the solder layer 110 _(N) after plating step 314. To make the solder layer in the wider openings 502 thicker without impacting the solder layers in the narrow openings 602, in step 316 a second photoresist layer is deposited over the first photoresist layer, the second photoresist layer covering at least the solder layers in the narrow openings. Alternatively, the second photoresist layer covers the entire first photoresist and openings are formed in the second photoresist layer to expose the solder layers in the wide openings of the first photoresist layer. Preferably, the diameter of the openings in the second photoresist layer is substantially the same as the diameter of the wide openings in the first photoresist layer but it is understood that the diameters might be different. Then in step 320 the wafer is placed back in the solder plating bath for a fourth time period to complete the plating of the solder layers in the wide openings to the desired height. The preceding two steps are illustrated in FIG. 8. A second photoresist layer 804 overlays the first photoresist layer 504, covering the narrow solder layers 110 _(N). Openings 802 are formed in the photoresist layer 804 to expose the solder layers 606 using, for example, a conventional plasma etch or a conventional liquid chemical etch that selectively removes the patterned photoresist 804. The diameter of openings 802 is approximately the same diameter as the openings 502 (FIG. 7) so that the diameter of openings 502 are not significantly changed when the openings 802 are formed since the photoresist layer 804 will extend into the opening 502 when deposited. Then, the wafer with device 102 thereon is placed back in the solder plating bath (not shown) for the fourth time period to form the solder layer 806. The result is the combined heights of the solder layers 606 and 806 are approximately equal to the height H_(S) of the solder layer 110 _(N). Thus, layers 606 and 806 together are referred to as solder layer 110 _(W).

Like the above calculation to determine the first time period, the third time period (t3) is chosen so that the narrow solder layers 110 _(N) have a desired height H_(S) after the first solder plating step 314. Then, assuming the plating conditions (e.g., plating current) are substantially the same during the first and second solder plating steps, the fourth time period, t4, is determined

t4≅t3(A_(W)/A_(S)−1);

where t3 and t4 are the third and fourth time periods, respectively, A_(W) is the area of each of the wide pillars or that of the wide openings, and A_(N) is the area of the narrow pillars or that of the narrow openings.

Alternatively and assuming the plating conditions (e.g., plating current) are substantially the same during the first and second solder plating steps, t4 can be calculated based on the amount of time T needed to plate the solder layers in the wide openings to the same height as the solder layers in the narrow openings plated in time period t3. Thus t4 is approximately T−t3. However, it is understood that if the plating conditions are different during the first solder plating step 314 from that in the second solder plating step 320, the fourth time period might be adjusted accordingly.

As stated above, the height of the pillars and the solder layer is proportional to the plating current and time used to form them. Knowing the diameter of the opening 502 allows the relatively precise control of the height of the pillars and solder layer during formation with an accuracy of approximately 10% or better.

Next, in step 322 the photoresist layers 504, 704 are removed by ashing using an oxygen plasma or by dipping the device 102 into a chemical stripping bath. Then the conductive layer 216, 218 exposed by the removed photoresist is removed in step 324 by plasma etching or by wet etching. The result is shown in FIG. 9 where two sets of joints are shown, a two wide joints 106 _(W) and two narrow joints 106 _(N), each having substantially the same height above the device 102.

In step 326, the wafer (not shown) is singulated into multiple devices 102. Then in step 328, each flip-chip device is bonded to a substrate using a conventional flip-chip bonding technique to form the package 100 shown in FIG. 1. Briefly, the package 100 is formed by bringing the flip-chip device 102 into proximity to the substrate such that the substrate pads 114 are aligned with respective metal pillars 108 on the flip-chip device. Then the metal pillars are then bonded to their respective substrate pads by melting the solder layers 110 so that the solder wets both the pillars and the substrate pads and then the device and substrate are cooled to solidify the solder.

Next, in step 330, the final steps to complete the packaging of the bonded device and substrate are done, such as forming an underfill layer between the device and the substrate, adding a heat spreader lid, forming an overmold of epoxy to the device and substrate for environmental protection, testing, package marking, etc.

In an alternative embodiment, instead of applying the solder to the ends of the copper pillars, a layer of solder is deposited on each of the substrate pads 114 by using a patterned solder mask (not shown) on the substrate 104 with the substrate pads exposed and the solder plated onto the exposed pads, using either conventional electroplating, or electroless plating. In this example, the solder layers 110 are not formed and steps 314 through 320 are not performed.

While the embodiments described above entail the formation of wide pillars before narrow pillars, three or more different pillar widths might be formed on a device using the concepts described above.

Although the elements in the following method claims are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence.

It is understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention might be made by those skilled in the art without departing from the scope of the invention as expressed in the following claims. 

1. A method comprising the steps of: A) providing, a wafer having a plurality of flip-chip devices, each flip-chip device having a plurality of die pads thereon; B) depositing a first layer of photoresist on the wafer; C) patterning the first layer of photoresist to form a first plurality of openings therein, each of the first openings having a first diameter and exposing a first set of die pads; D) plating metal into the first openings to form a first metal pillar in each of the first openings for a first time period; E) patterning the first layer of photoresist to form a second plurality of openings therein, each of the second openings having a second diameter and exposing a second set of die pads; and F) plating metal into the first and second openings for a second time period to add to each of the metal pillars in the first openings and form a metal pillar in each of the second openings; wherein the first diameter is greater than the second diameter, the first set of die pads is different from the second set of die pads, and the first and second time periods and plating conditions are chosen such that, after step F) each of the pillars in the second openings has substantially the same height as a pillars in each of the first openings.
 2. The method of claim 1 wherein the plated metal is copper.
 3. The method of claim 1 further comprising the steps of: G) plating, after step F), solder into the first and second openings to form a solder layer in each of the openings and on exposed ends of the metal pillars; H) depositing a second layer of photoresist to at least cover the second openings in the first photoresist layer; and I) plating solder into the first openings to form a second solder layer in each of the first openings and on the first solder layer therein.
 4. The method of claim 3 wherein each of the metal pillars of the first and second plurality of metal pillars has a height above a die pad and each of the solder layers has a height above its respective metal pillar, and a ratio of the height of a metal pillar to the height of its respective solder layer is 1:10 to 100:1.
 5. The method of claim 4 wherein the height of each of the metal pillars is 20 to 70 microns, the height of each solder layers is 10 to 60 microns, and the first and second diameters range from 20 to 80 microns.
 6. The method of claim 4 wherein a sum of the height of each of the metal pillars and its respective solder layer is between 5 microns and 130 microns and the first and second diameters range from substantially 5 to 150 microns.
 7. The method of claim 4 wherein the first diameter is less than 80 microns and the second diameter is greater than 80 microns, and a sum of the height of each metal pillar and its respective solder layer is 80 microns or less.
 8. The method of claim 3 wherein in step H) comprises the steps of: depositing a second layer of photoresist covering the first photoresist layer; patterning the second photoresist layer form a plurality of openings therein, the openings having a diameter substantially equal to the first diameter and exposing the solder layers in the first openings of the first photoresist layer.
 9. The method of claim 1 further comprising the step of: depositing, before step B), a conductive layer over the plurality of die pads; wherein in steps D) and G) the plating is by electroplating using the conductive layer as a electroplating electrode.
 10. The method of claim 9 wherein the conductive layer comprises a barrier layer and a strike layer deposited over the barrier layer.
 11. The method of claim 10 wherein the barrier comprises titanium and the strike layer comprises copper.
 12. The method of claim 1 further comprising the step of removing, after step F), the first photoresist layer.
 13. A method of claim 12 further comprising the steps of: singulating the wafer to separate the plurality of a flip-chip devices into individual devices; selecting one of the singulated devices; providing a substrate having a plurality of substrate pads, each of the plurality of substrate pads positioned on the substrate to align with a respective one of the metal pillars of the first and second plurality of metal pillars on the selected flip-chip device; bringing the selected flip-chip device in proximity to the substrate such that all the plurality of substrate pads positioned on the substrate are aligned with a respective one of the metal pillars on the selected flip-chip device; and bonding the metal pillars to their respective substrate pads using solder to form a package.
 14. The method of claim 13 wherein the flip-chip device comprises a material selected from the group consisting of silicon, gallium arsenide, indium phosphide, and a combination thereof and wherein the substrate is selected from the group consisting of glass-epoxy, polytetrafluoroethylene, ceramic, silicon, glass, and a combination thereof.
 15. The method of claim 13 further comprising the steps of: forming, after the bonding step, an underfill layer between the flip-chip device and the substrate; and forming, after forming the underfill layer, an overmold on the flip-chip device and the substrate.
 16. A method comprising the steps of: A) providing a wafer having a plurality of flip-chip devices, each flip-chip device having a plurality of die pads thereon; B) depositing a first layer of photoresist on the wafer; C) patterning the first layer of photoresist to form a first plurality of openings therein, each of the first openings having a first diameter and exposing a first set of die pads; D) plating metal into the first plurality of openings to form a first metal pillar in each of the first plurality of openings, each of the first metal pillars having substantially the first diameter and a first height above its respective die pad; E) patterning the first layer of photoresist to form a second plurality of openings therein, each of the second openings having a second diameter and exposing a second set of die pads; and F) plating metal into the first and second openings and onto the first metal pillars and exposed second set of die pads, respectively, to form a second metal pillar in each of the first openings and a metal pillar in each of the second openings, each of the metal pillars in the second openings having substantially the second diameter and a height above its respective die pad, and each of the second pillars in the first plurality of openings having substantially the first diameter and a height above its respective first metal pillar; wherein the first diameter is greater than the second diameter, a sum of the first and second pillar heights in each of the first openings is substantially equal to the height of the metal pillar in each of the second openings, and the first set of die pads is different from the second set of die pads.
 17. The method of claim 16 wherein each of the copper pillars of the first and second plurality of copper pillars has a height above a die pad and each of the solder layers has a height above its respective copper pillar, and the height of each of the copper pillars is 20 to 70 microns, the height of each solder layers is 10 to 60 microns, a sum of the height of each copper pillar and its respective solder layer is 80 microns or less, and the first diameter is less than 80 microns and the second diameter is greater than 80 microns.
 18. The method of claim 16 wherein each of the copper pillars of the first and second plurality of copper pillars has a height above a die pad and each of the solder layers has a height above its respective copper pillar, and a sum of the height of each of the copper pillars and its respective solder layer is between 5 microns and 130 microns and the first and second diameters range from substantially 5 to 150 microns.
 19. The method of claim 16 wherein the barrier comprises titanium and the strike layer comprises copper.
 20. The method of claim 15 further comprising the steps of: forming, after the bonding step, an underfill layer between the flip-chip device and the substrate; and forming, after forming the underfill layer, an overmold on the flip-chip device and the substrate. 